Many surgical, diagnostic, therapeutic and prophylactic medical procedures require the placement of objects such as sensors, implants, tubes, catheters and treatment devices within the body. These procedures cover a large spectrum including, for example: a) placement of tubes to facilitate the delivery of drugs, nutrition or other fluids into a patient's circulatory or digestive system, b) placement of tubes to facilitate respiration of a patient, c) placement of tubes to facilitate removal of gastrointestinal tract contents for analysis and/or treatment, d) insertion of probes or surgical devices for diagnostic or therapeutic treatments, and e) placement of orthopedic devices such as artificial hips and knees.
In some instances, placement of a device is for a limited time, such as during surgery or catheterization. In other situations, such as with feeding tubes or orthopedic implants, the devices are to be used for long-term treatment. It is beneficial to provide real-time information for accurately determining the location and orientation of objects both during and after implantation within a patient's body, preferably without using X-ray imaging.
Noninvasive techniques for determining the location and orientation of implanted devices are often preferred, as such techniques are generally more comfortable for the patient and easier to perform. Field sensors, such as Hall effect devices, coils or antennae, have been included in medical devices to allow for noninvasive monitoring of the position of the devices.
PCT Patent Publication WO 96/05768 to Ben-Haim et al. and corresponding U.S. patent application Ser. No. 08/793,371, which are assigned to the assignee of the present patent application and which are incorporated herein by reference, describe a locating system for determining the location and orientation of an invasive medical instrument, whereby an externally-applied RF field induces a current in three coils located within the invasive medical instrument. Wires or some other form of physical leads are used to carry this induced signal from the catheter to a signal processor in the extrabody space. The processor analyzes the signal so as to calculate the location and orientation of the invasive medical instrument.
In many applications, a wireless passive emitter, or “tag,” is affixed to a device that is inserted into the body. Such a tag contains no internal power source, but is rather actuated by an external energy field, typically applied from outside the body. The tag then emits electromagnetic energy, which is detected by antennas or other sensors outside the body. The detected signals are generally used simply to ascertain the presence of the tag within a given region (such as the abdominal cavity), although some tags may also be used to determine position coordinates. Some passive tags receive and re-emit electromagnetic radiation, typically with a frequency and/or phase shift.
For example, U.S. Pat. No. 6,026,818 to Blair et al., whose disclosure is incorporated herein by reference, describes a method and device for the detection of unwanted objects in surgical sites, based on a medically-inert detection tag which is affixed to objects such as medical sponges or other items used in body cavities during surgery. The detection tag contains a single signal emitter, such as a miniature ferrite rod and coil and capacitor element embedded therein. Alternatively, the tag includes a flexible thread composed of a single loop wire and capacitor element. A detection device is utilized to locate the tag by pulsed emission of a wide-band transmission signal. The tag resonates with a radiated signal, in response to the wide-band transmission, at its own single non-predetermined frequency, within the wide-band range. The return signals build up in intensity at a single (though not predefined) detectable frequency over ambient noise, so as to provide recognizable detection signals.
U.S. Pat. No. 5,057,095 to Fabian, whose disclosure is incorporated herein by reference, describes apparatus for detecting a surgical implement in human or animal tissue, comprising a detector that is responsive to the presence, within an interrogation zone, of a surgical implement to which a marker is secured. The marker is adapted to produce identifying signal characteristics within a frequency band generated in the interrogation zone. Variations in the phase and or direction of the interrogating field and changes in the electromagnetic coupling between markers and receiver are intended to optimize coupling therebetween.
U.S. Pat. No. 6,076,007 to England et al., whose disclosure is incorporated herein by reference, describes a method for determining the position and orientation of a surgical device within a human body. In one application, a catheter or prosthesis is characterized in that it carries, at a predetermined location, a tag formed of a high permeability, low coercivity magnetic material. The position of the tag (and hence of the surgical device) is sensed by remotely detecting its magnetic response to an interrogating signal.
U.S. Pat. No. 5,325,873 to Hirschi et al., whose disclosure is incorporated herein by reference, describes a system to verify the location of a tube or other object inserted into the body. It incorporates a resonant electrical circuit attached to the object which resonates upon stimulation by a hand-held RF transmitter/receiver external to the body. The electromagnetic field generated due to resonance of the circuit is detected by the hand-held device, which subsequently turns on a series of LEDs to indicate to the user the direction to the target. An additional visual display indicates when the transmitter/receiver is directly above the object.
Passive sensors and transponders, fixed to implanted devices, can also be used for conveying other diagnostic information to receivers outside the body. For example, U.S. Pat. No. 6,053,873 to Govari et al., whose disclosure is incorporated herein by reference, describes a stent adapted for measuring a fluid flow in the body of a subject. The stent contains a coil, which receives energy from an electromagnetic field irradiating the body so as to power a transmitter for transmitting a pressure-dependent signal to a receiver outside the body.
As another example, U.S. Pat. No. 6,206,835 to Spillman et al., whose disclosure is incorporated herein by reference, describes an implant device that includes an integral, electrically-passive sensing circuit, communicating with an external interrogation circuit. The sensing circuit includes an inductive element and has a frequency-dependent variable impedance loading effect on the interrogation circuit, varying in relation to the sensed parameter.
U.S. Pat. Nos. 5,391,199 and 5,443,489 to Ben-Haim, whose disclosures are incorporated herein by reference, describe systems wherein the coordinates of an intrabody probe are determined using one or more field sensors, such as a Hall effect device, coils, or other antennae carried on the probe. Such systems are used for generating three-dimensional location information regarding a medical probe or catheter. Preferably, a sensor coil is placed in the catheter and generates signals in response to externally-applied electromagnetic fields. The electromagnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
U.S. Pat. No. 6,198,963 to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes apparatus for confirmation of intrabody tube location. The initial location of the object is determined as a reference point, and subsequent measurements are made to determine whether the object has remained in its initial position. Measurements are based upon one or more signals transmitted to and/or from a sensor fixed to the body of the object whose location is being determined. The signal could be ultrasound waves, ultraviolet waves, radio frequency (RF) waves, or static or rotating electromagnetic fields.
Other devices comprise multiple transducers which each perform specific tasks. For example, U.S. Pat. No. 6,239,724 to Doron et al., which is incorporated herein by reference, describes an implantable telemetry device that contains one transducer for converting a power signal from outside the body into electrical power for the device, a second transducer for receiving a position field signal form outside the body, and a third transducer for transmitting a signal to a site outside the body.
U.S. Pat. No. 6,172,499 to Ashe, whose disclosure is incorporated herein by reference, describes a device for measuring the location and orientation in the six degrees of freedom of a receiving antenna with respect to a transmitting antenna utilizing multiple-frequency AC magnetic signals. The transmitting component consists of two or more transmitting antennae of known location and orientation relative to one another. The transmitting antennae are driven simultaneously by AC excitation, with each antenna occupying one or more unique positions in the frequency spectrum. The receiving antennae measure the transmitted AC magnetic field plus distortions caused by conductive metals. A computer then extracts the distortion component and removes it from the received signals, providing the correct position and orientation output.
U.S. Pat. No. 6,261,247 to Ishikawa et al., which is incorporated herein by reference, describes an implantable position sensing system comprising a plurality of spherical semiconductors, which are capable of determining their relative positions and communicating this information among each other and to an external processing unit. Radio frequency signals are used for communication and to power the implanted spherical semiconductors.
Current intrabody radio frequency powered position sensors are limited in the amount of power they can receive, due to their typically small size, as induced current varies with cross-sectional area of the receiving coil. Additional power would be desirable, e.g., so that additional computations can be performed by the sensor and/or to increase the strength of the signal transmitted by the sensor.